The present application relates generally to electric machine control. More specifically, control schemes and controller designs are described that pulse the operation of an electric machine during selected operating conditions to facilitate operating the electric machine in a more energy efficient manner.
The phrase “electric machine” as used herein is intended to be broadly construed to mean both electric motors and generators. Electric motors and generators are structurally very similar. When an electric machine is operating as motor, it converts electrical energy into mechanical energy. When operating as a generator, the electric machine converts mechanical energy into electrical energy.
Electric motors and generators are used in a very wide variety of applications and under a wide variety of operating conditions. In general, many modern electric machines have relatively high energy conversion efficiencies. However, the energy conversion efficiency of most electric machines can vary considerably based on their operational load. Many applications require that the electric machine operate under a wide variety of different operating load conditions, which means that the electric machine often doesn't operate as efficiently as it is capable of. The nature of this problem is illustrated in FIG. 1, which is a motor efficiency map 10 that diagrammatically shows the efficiency of a representative motor under different operating conditions. More specifically, the figure plots the energy conversion efficiency of the motor as a function of motor speed (the X-axis) and torque generated (the Y-axis).
As can be seen in FIG. 1, the illustrated motor is generally most efficient when it is operating within a particular speed range and generating torque within a defined range. For the particular motor shown, the most efficient region of its operating range is the operating region labeled 14 which is generally in the range of 4500-6000 RPM with a torque output in the range of about 40-70 Nm where its energy conversion efficiency is approximately 96%. The region 14 is sometimes referred to herein as the “sweet spot”, which is simply the motor's most efficient operating region.
As can be seen in FIG. 1, at any particular motor speed, there will be a corresponding most efficient output torque which is diagrammatically illustrated by maximum efficiency curve 16. For any given motor speed, the motor's efficiency tends to drop off somewhat when the motor's load is higher or lower than the most efficient load. In some regions the motor's efficiency tends to drop relatively quickly, as for example when the torque output falls below about 30 Nm in the illustrated motor.
If the operating conditions could be controlled so that the motor is almost always operated at or near its sweet spot, the energy conversion efficiency of the motor would be quite good. However, many applications require that the motor operate over a wide variety of load conditions with widely varying torque requirements and widely varying motor speeds. One such application that is easy to visualize is automotive and other vehicle or mobility applications where the motor speed may vary between zero when the vehicle is stopped to a relatively high RPM when cruising at highway speeds. The torque requirements may also vary widely at any of those speeds based on factors such as whether the vehicle is accelerating or decelerating, going uphill, downhill, going on relatively flat terrain, etc., the weight of the vehicle and many other factors. Of course, motors used in other applications may be subjected to a wide variety of operating conditions as well.
Although the energy conversion efficiency of conventional electric machines is generally good, there are continuing efforts to further improve energy conversion efficiencies over broader ranges of operating conditions.